In communication networks, it is known to use pseudowire (PW) technology for establishing connections over a packet-switched network. In this connection, a PW may be regarded as an emulation of a point-to-point connection over the packet-switched network. PW technology is subject to various standards, e.g., Internet Engineering Task Force (IETF) RFC 3985.
For scenarios where resiliency is needed, PW redundancy networks may be used. In a PW redundancy network, a plurality of PW may be provided in a redundant manner so that in the case of a fault one PW can be replaced by another. Proposals for a PW resiliency architecture using PW redundancy can be found in the IETF draft entitled “Pseudowire Redundancy”, draft-ietf-pwe3-redundancy-06 dated Feb. 16, 2012.
In a PW redundancy network, mechanisms for selecting between multiple redundant PWs are needed. In this connection, the IETF draft entitled “Pseudowire Preferential Forwarding Status Bit”, draft-ietf-pwe3-redundancy-bit-06, dated Feb. 27, 2012 defines a “Preferential Forwarding” status bit by means of which a Provider Edge (PE) node on one end of a PW can indicate the preferential forwarding state of the PW to a remote PE at the other end of the PW. According to this IETF draft, an independent mode of operation may be used for PW selection. In the independent mode, a PW is selected as “active”, i.e., used for forwarding user traffic, when the Preferential Forwarding status bit indicates that the preferential forwarding status is active at both PE nodes. For cases in which this condition is met by multiple PWs, local precedence parameters may be configured in the PE nodes so that a single active PW may be selected. In independent mode, the precedence parameters apply to PWs for which the preferential forwarding status was already determined to be active.
A PW redundancy architecture working according to the above principles is schematically illustrated in FIG. 1. The PW redundancy architecture of FIG. 1 includes PE nodes 21, 22, 23, 24, also denoted as PE1, PE2, PE3, PE4, and PWs PW13, PW14, PW23, PW24 provided between the PE nodes 21, 22, 23, 24. More specifically, the PW PW13 extends between the PE node 21 and the PE node 23, the PW PW14 extends between the PE node 21 and the PE node 24, the PW PW23 extends between the PE node 22 and the PE node 23, and the PW PW24 extends between the PE node 22 and the PE node 24. Further, the PW redundancy architecture of FIG. 1 includes an attachment circuit (AC) 31 connected to the PE node 21, an AC 32 connected to the PE node 22, an AC 33 connected to the PE node 23, and an AC 34 connected to the PE node 24.
In the PW redundancy architecture of FIG. 1, one of the ACs 31, 32 and one of the ACs 33, 34 is selected first. This selection may be performed by Customer Edge (CE) nodes coupled to the ACs 31, 32, 33, 34. In the scenario of FIG. 1, it is assumed that the upper ones, e.g., AC 31 and AC 33 are selected. The PE node that is attached to the selected AC, in the illustrated scenario the PE node 21 and the PE node 23, sets the Preferential Forwarding status bit as active for both of its attached PWs. In the illustrated scenario, the PE node 21 sets the Preferential Forwarding status bit for the PW PW13 and for the PW PW13 as active. Similarly, the PE node 23 sets the Preferential Forwarding status bit for the PW PW13 and for the PW PW23 as active. The PE node attached to a non-selected AC, in the illustrated scenario the PE node 22 and the PE node 24, sets the Preferential Forwarding status bit as “standby”, i.e., not used for forwarding user traffic, for all attached PWs. In the illustrated scenario, the PE node 22 sets the Preferential Forwarding status bit for the PW PW23 and for the PW PW24 as standby. Similarly, the PE node 24 sets the Preferential Forwarding status bit for the PW PW14 and for the PW24 as standby. In the illustrated scenario, due to the above criterion for selecting a PW as active in the independent mode, forwarding of traffic will only be enabled on the PW PW 13 for which both the PE node 21 and the PE node 23 have set the Preferential Forwarding status bit to declare the preferential forwarding status as active.
In order to provide resiliency in the PW redundancy architecture of FIG. 1, communication between PE nodes of the same side is needed, e.g., between PE node 21 and PE node 22 on the left side or between PE node 23 and PE node 24 on the right side. For example, if PE node 23 fails in the scenario of FIG. 1, PE node 24 needs to declare the preferential forwarding status of its attached PWs, i.e., the PWs PW14 and PW24, as active. Consequently, the PE node 24 needs to be aware of the status of the PE node 23 or the AC 33. Such communication between PE nodes on the same side of a PE redundancy network increases complexity of implementation.
In view of the above, there is a need for techniques which allow for efficiently implementing PW selection in a PW redundancy network.